1. Field of the Invention
The present invention relates generally to apparatus and methods for removing occluding material from stented regions within blood vessels which have restenosed. More particularly, the present invention relates to apparatus and methods for sensing a stent within the wall of a restenosed blood vessel and removing the occluding material without damaging the stent.
Percutaneous transluminal angioplasty (PTA) and percutaneous transluminal coronary angioplasty (PTCA) procedures are widely used for treating stenotic atherosclerotic regions of a patient's vasculature to restore adequate blood flow. Catheters having an expansible distal end, typically in the form of an inflatable balloon, are positioned in an artery, such as a coronary artery, at a stenotic site. The expansible end is then expanded to dilate the artery in order to restore adequate blood flow to regions beyond the stenosis. While PTA and PTCA have gained wide acceptance, these angioplasty procedures suffer from two major problems: abrupt closure and restenosis.
Abrupt closure refers to rapid reocclusion of the vessel within hours of the initial treatment, and often occurs in patients who have recently suffered acute myocardial infarction. Abrupt closure often results from either an intimal dissection or from rapid thrombus formation which occurs in response to injury of the vascular wall from the initial angioplasty procedure. Restenosis refers to a re-narrowing of the artery over the weeks or months following an initially apparently successful angioplasty procedure. Restenosis occurs in up to 50% of all angioplasty patients and results at least in part from smooth muscle cell proliferation and migration.
Many different strategies have been proposed to ameliorate abrupt closure and reduce the rate of restenosis. Of particular interest to the present invention, the implantation of vascular stents following angioplasty has become widespread. Stents are thin-walled tubular scaffolds which are expanded in the arterial lumen following the angioplasty procedure. Most commonly, the stents are formed from a malleable material, such as stainless steel, and are expanded in situ using a balloon. Alternatively, the stents may be formed from a shape memory alloy or other elastic material, in which case they are allowed to self-expand at the angioplasty treatment site. In either case, the stent acts as a mechanical support for the artery wall, inhibiting abrupt closure and reducing the restenosis rate as compared to PTCA.
While stents have been very successful in inhibiting abrupt closure and reasonably successful in inhibiting restenosis, a significant portion of the treated patient population still experiences restenosis over time. Most stent structures comprise an open lattice, typically in a diamond or spiral pattern, and cell proliferation (also referred to as intimal hyperplasia) can intrude through the interstices between the support elements of the lattice. As a result, instead of forming a barrier to hyperplasia and restenosis, the stent can become embedded within an accumulated mass of thrombus and tissue growth, and the treatment site once again becomes occluded.
To date, proposed treatments for restenosis within previously stented regions of the coronary and other arteries have included both follow-up balloon angioplasty and directional atherectomy, e.g. using the Simpson directional atherectomy catheter available from Guidant Corporation, Santa Clara, Calif. Neither approach has been wholly successful. Balloon angioplasty can temporarily open the arterial lumen, but rarely provides long-term patency. Directional atherectomy can successfully debulk the lumen within the stent, but typically does not fully restore the stented lumen to its previous diameter because the catheter removes the stenotic material in an asymmetric pattern. Moreover, it has been found that the atherectomy cutting blades can damage the implanted stent. Such adverse effects were reported by Bowerman et al. in Disruption of a coronary stent during atherectomy for restenosis in the December 1991 issue of Catheterization and Cardiovascular Diagnosis and by Meyer et al. in Stent wire cutting during coronary directional atherectomy in the May 1993 issue of Clinical Cardiology. The possibility of such adverse outcomes is likely to limit the application of atherectomy as a treatment for stent restenosis and will probably result in more tentative use of the atherectomy cutter within the stented region when it is applied, leading to less complete removal of the stenosis.
For these reasons, it would be desirable to provide improved methods for treating restenosis within regions of the vasculature which have previously been implanted with stents. More particularly, it would be desirable to provide an apparatus for removal of stenotic material from within a stent which includes a sensing means for sensing when the stenosis removal mechanism is approaching or contacting the stent within the arterial wall so that the occluded artery can be effectively recanalized without damaging the stent. The stenosis removal mechanism of the apparatus may advantageously be a directional cutting or debulking device for selectively removing the stenotic material from within a stent or it may be a symmetrical cutting or debulking device for removing the stenotic material uniformly from the entire inner periphery of the stent. 2. Description of the Background Art
Post-angioplasty restenosis is discussed in the following publications: Khanolkar (1996) Indian Heart J. 48:281-282; Ghannem et al. (1996) Ann. Cardiol. Angeiol. 45:287-290; Macander et al. (1994) Cathet. Cardiovasc. Diagn. 32:125-131; Strauss et al. (1992) J. Am. Coll. Cardiol. 20:1465-1473; Bowerman et al. (1991) Cathet. Cardiovasc. Diagn. 24:248-251; Moris et al. (1996) Am. Heart. J. 131:834-836; Schomig et al. (1994) J. Am. Coll. Cardiol. 23:1053-1060; Haude et al., "Treatment of In-Stent Restenosis," in Chapter 52, pages 357-365; Gordon et al. (1993) J. Am. Coll. Cardiol. 21:1166-1174; and Baim et al. (1993) Am. J. Cardiol. 71:364-366. These publications include descriptions of follow-up angioplasty and atherectomy as possible treatments for restenosis.
Atherectomy catheters having ultrasonic imaging transducers are described in U.S. Pat. Nos. 5,000,185 and 5,100,424. Rotary ablation catheters having selectively expandable burr components are described in U.S. Pat. Nos. 5,217,474 and 5,308,354. A catheter carrying an expandable filter is described in U.S. Pat. No. 4,723,549.
Thrombectomy and atherectomy catheters having rotating brush and filament structures are described in U.S. Pat. Nos. 5,578,018; 5,535,756; 5,427,115; 5,370,653; 5,009,659; and 4,850,957; WO 95/29626; DE 39 21 071 C2; and Netherlands 9400027.
Representative atherectomy catheters are described in U.S. Pat. Nos. 4,273,128; 4,445,509; 4,653,496; 4,696,667; 4,706,671; 4,728,319; 4,732,154; 4,762,130; 4,790,812; 4,819,634; 4,842,579; 4,857,045; 4,857,046; 4,867,156; 4,883,458; 4,886,061; 4,890,611; 4,894,051; 4,895,560; 4,926,858; 4,966,604; 4,979,939; 4,979,951; 5,011,488; 5,011,489; 5,011,490; 5,041,082; 5,047,040; 5,071,424; 5,078,723; 5,085,662; 5,087,265; 5,116,352; 5,135,483; 5,154,724; 5,158,564; 5,160,342; 5,176,693; 5,192,291; 5,195,954; 5,196,024; 5,209,749; 5,224,945; 5,234,451; 5,269,751; 5,314,438; 5,318,576; 5,320,634; 5,334,211; 5,356,418; 5,360,432; 5,376,100; 5,402,790; 5,443,443; 5,490,859; 5,527,326; 5,540,707; 5,556,405; 5,556,408; and 5,554,163.
The disclosures of these patent are incorporated herein by reference in their entirety.